Dental erosion involves demineralization and damage to the tooth structure due to acid attack from nonbacterial sources. Erosion is found initially in the enamel and, if unchecked, may proceed to the underlying dentin. Dental erosion may be caused or exacerbated by acidic foods and drinks, exposure to chlorinated swimming pool water, and regurgitation of gastric acids. The tooth enamel is a negatively charged surface, which naturally tends to attract positively charged ions such as hydrogen and calcium ions, while resisting negatively charged ions such as fluoride ions. Depending upon relative pH of surrounding saliva, the tooth enamel will lose or gain positively charged ions such as calcium ions. Generally saliva has a pH between 7.2 to 7.4. When the pH is lowered and concentration of hydrogen ions becomes relatively high, the hydrogen ions will replace the calcium ions in the enamel, forming hydrogen phosphate (phosphoric acid), which damages the enamel and creates a porous, sponge-like roughened surface. If saliva remains acidic over an extended period, then remineralization may not occur, and the tooth will continue to lose minerals, causing the tooth to weaken and ultimately to lose structure.
Dentinal hypersensitivity is acute, localized tooth pain in response to physical stimulation of the dentine surface as by thermal (hot or cold) osmotic, tactile combination of thermal, osmotic and tactile stimulation of the exposed dentin. Exposure of the dentine, which is generally due to recession of the gums, or loss of enamel, frequently leads to hypersensitivity. Dentinal tubules open to the surface have a high correlation with dentine hypersensitivity. Dentinal tubules lead from the pulp to the cementum. When the surface cementum of the tooth root is eroded, the dentinal tubules become exposed to the external environment. The exposed dentinal tubules provide a pathway for transmission of fluid flow to the pulpal nerves, the transmission induced by changes in temperature, pressure and ionic gradients.
Heavy metal ions, such as zinc, are resistant to acid attack. Zinc ranks above hydrogen in the electrochemical series, so that metallic zinc in an acidic solution will react to liberate hydrogen gas as the zinc passes into solution to form di-cations, Zn2+. Zinc has been shown to have antibacterial properties in plaque and caries studies.
Soluble zinc salts, such as zinc citrate, have been used in dentifrice compositions, see, e.g., U.S. Pat. No. 6,121,315, but have several disadvantages. Zinc ions in solution impart an unpleasant, astringent mouthfeel, so formulations that provide effective levels of zinc, and also have acceptable organoleptic properties, have been difficult to achieve. Finally, the zinc ions will react with anionic surfactants such as sodium lauryl sulfate, thus interfering with foaming and cleaning. Zinc oxide and insoluble zinc salts, on the other hand, may do a poor job of delivering zinc to the teeth because of their insolubility.
While the prior art discloses the use of various oral compositions for the treatment of dentinal hypersensitivity, dental caries, and enamel erosion and demineralization, there is still a need for additional compositions and methods which provide improved performance in such treatments.